lake albert salinity reduction study - scenario

Lake Albert Salinity Reduction Study - Scenario Schematisation and Testing

Reference: R.N20056.003.02_LakeAlbert_Meshing&1yrScenarios.docx

Date: March 2014 Confidential

A part of BMT in Energy and Environment

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Lake Albert Salinity Reduction Study – Scenario Schematisation and Testing

Prepared for: South Australian Department of Environment, Water and Natural Resources

Prepared by: BMT WBM Pty Ltd (Member of the BMT group of companies)

Offices Brisbane Denver London Mackay Melbourne Newcastle Perth Sydney Vancouver


Document Control Sheet

BMT WBM Pty Ltd 126 Belford Street BROADMEADOW NSW 2292 Australia PO Box 266 Broadmeadow NSW 2292 Tel: +61 2 4940 8882 Fax: +61 2 4940 8887 ABN 54 010 830 421 www.bmtwbm.com.au

Document: R.N20056.003.02_LakeAlbert_Meshing&1yrScenarios.docx

Title: Lake Albert Salinity Reduction Study - Scenario Schematisation and Testing

Project Manager: Rohan Hudson

Author: Rohan Hudson

Client: South Australian Department of Environment, Water and Natural Resources

Client Contact: Theresa Andrew (nee Myburgh)

Client Reference: n/a

Synopsis: BMT WBM was commissioned by the South Australian Department of Environment, Water and Natural Resources (DEWNR) to undertake a range of studies aimed at improving the understanding of salinity transport and mixing mechanisms in Lake Albert. A numerical model capable of simulating salinity dynamics of the Lower Lakes and Coorong was used to evaluate the effectiveness of six potential management options designed to reduce salinity levels in Lake Albert.

VERSION/CHECKING HISTORY

Version Number Date Checked by Issued by

1 (draft) 31/1/2014 LJK

RMH 31/1/2014

2 (final) 6/3/2014 RMH 6/3/2014

DISTRIBUTION

Destination VERSION

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BMT WBM File

BMT WBM Library

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pdf

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http://www.bmtwbm.com.au/

Lake Albert Salinity Reduction Study - Scenario Schematisation and Testing i

Contents


Contents

1 Introduction 1

1.1 Summary of Associated Studies and Reports 1

1.1.1 Lake Albert Salinity Reduction Study - Preliminary Investigations (BMT WBM, 2013a) 1

1.1.2 Lake Albert Salinity Reduction Study – Model Setup and Calibration Report (BMT WBM, 2013b) 2

1.1.3 Lake Albert Salinity Reduction Study – Wind Mixing Investigation 2

1.1.4 Lake Albert Salinity Reduction Study – Model Scenario Schematisation and Initial Testing 3

1.1.5 Lake Albert Salinity Reduction Study – Three Year Scenario Model Investigations 3

1.2 Structure of Report 4

2 Model Setup and Description of Scenario Simulations 5

2.1 Model Configuration 5

2.2 Model Schematisation of Six Management Options 5

2.3 Description of One Year Forecast Scenario Simulations 6

2.4 Boundary Conditions for Scenario Simulations 6

2.4.1 River Murray Inflows 7

2.4.2 Net Rainfall – Evaporation 8

2.4.3 Pelican Point Wind Data 9

2.4.4 Victor Harbour Tides 10

2.4.5 Offshore Wave Data (BMT ARGOSS) 11

2.4.6 Salt Creek Inflows 11

2.4.7 Catchment Inflows 12

2.4.8 Use of Automated Barrage Opening Logic and Target Lake Water Levels 12

2.4.9 Adopted Model Barrage Structures 13

2.4.10 Target Management Lake Water Levels (Barrage Openings) 13

2.4.11 Adopted Barrage Operating Rules 14

2.5 Model Initial Conditions 15

2.5.1 Initial Water Levels 15

2.5.2 Initial Salinity and Salt Mass 15

2.5.3 Murray Mouth Bathymetry 16

2.6 Model Scenario Schematisation for Initial Assessment 19

2.6.1 Model Mesh Development for Scenarios 19

2.6.2 Details of Narrung Narrows Dredging Scenario 21

2.6.3 Details of Coorong Connector Scenario 21

Lake Albert Salinity Reduction Study - Scenario Schematisation and Testing ii

Contents


2.6.4 Details of Permanent Narrung Control Structure 25

3 Model Output and Results 27

3.1 Description of Model Output 27

3.1.1 Water Level Output 27

3.1.2 Salinity Outputs 27

3.1.3 Water Volume and Salt Mass Change 27

3.2 Comparison of Management Option Results 29

3.2.1 Water Levels 29

3.2.2 Salinity (Salt Concentration) 29

3.2.3 Cumulative Salt Mass Change 35

3.2.4 Narrung Flows and Cumulative Volume Change 36

3.2.5 Coorong Connector Flow Summary 36

3.3 Sensitivity Testing of Scenarios 40

3.3.1 Coorong Connector Channel Conveyance 40

3.3.2 Quarterly Lake Cycling Performance 41

3.3.3 Permanent Narrung Structure Options 41

4 Conclusion 44

5 References 45

List of Figures

Figure 2-1 Wellington Inflow Data 7

Figure 2-2 Lower Lakes Rainfall and Evaporation Data 8

Figure 2-3 Pelican Point Wind Speed and Direction 9

Figure 2-4 Victor Harbour Tidal Water Level Data 10

Figure 2-5 Salt Creek Inflow Data 11

Figure 2-6 Finniss and Currency Creek Inflow Data 12

Figure 2-7 Standard and Cycled Target Lake Water Levels 14

Figure 2-8 Initial Model Salinity (ppt) and Transect Locations 17

Figure 2-9 Initial Murray Mouth Model Bathymetry, 18th April 2012 18

Figure 2-10 TUFLOW-FV Management Scenario Mesh 20

Figure 2-11 Base Case Mesh Narrung Narrows Bathymetry 22

Figure 2-12 Dredged Scenario Mesh Narrung Narrows Bathymetry 23

Figure 2-13 Schematisation of Coorong Connector 24

Figure 2-14 Proposed Location of Permanent Narrung Control Structure (Source: SKM) 25

Figure 2-15 Narrung Control Structure One Week Gate Closure Sequence 26

Figure 2-16 Narrung Control Structure Two Week Gate Closure Sequence 26

Lake Albert Salinity Reduction Study - Scenario Schematisation and Testing iii

Contents


Figure 3-1 Map of Model Output Locations 28

Figure 3-2 Modelled Water Levels in Lake Alexandrina for Proposed Scenarios 31

Figure 3-3 Modelled Water Levels in Lake Albert for Proposed Scenarios 31

Figure 3-4 Modelled Water Levels in the North Lagoon for Proposed Scenarios 32

Figure 3-5 Modelled Water Levels in the South Lagoon for Proposed Scenarios 32

Figure 3-6 Modelled Salinity in Lake Albert for Proposed Scenarios 33

Figure 3-7 Modelled Salinity in Lake Alexandrina for Proposed Scenarios 33

Figure 3-8 Modelled Salinity in the North Lagoon for Proposed Scenarios 34

Figure 3-9 Modelled Salinity in the South Lagoon for Proposed Scenarios 34

Figure 3-10 Modelled Lake Albert Salt Mass Change (Tonnes) for Six Scenarios 37

Figure 3-11 Modelled Lake Albert Percentage Salt Mass Change for Six Scenarios 37

Figure 3-12 Modelled Lake Albert Volume Change for Six Scenarios 38

Figure 3-13 Modelled Narrung Flows for Six Scenarios 38

Figure 3-14 Modelled Narrung Flows for Six Scenarios (six weeks) 39

Figure 3-15 Applied Wind Data (six weeks) 39

Figure 3-16 Coorong Connector Channel Modelled Flow Time series 40

Figure 3-17 Modelled Lake Albert Salt Mass Change (Tonnes) Sensitivity Test Results 42

Figure 3-18 Standard and Cycled Target Lake Water Levels 43

Figure 3-19 Target and Modelled Lake Water Levels (Quarterly Cycling) 43

List of Tables

Table 2-1 Adopted Hydraulic Properties for Barrages 13

Table 2-2 Adopted Gate Opening Rules for Barrages 14

Table 2-3 Initial Water Levels (18th April, 2012) 15

Table 2-4 Initial Salinity Assumptions 16

Table 2-5 Approximate Initial Salt Mass (18th April, 2012) 16

Table 3-1 Summary of Output Locations 27

Table 3-2 Summary of Lake Albert Salinity Results 30

Table 3-3 Summary of Lake Albert Salt Mass Change Results 35

Table 3-4 Summary of Lake Albert Sensitivity Test, Salinity Results 41

Table 3-5 Summary of Lake Albert Sensitivity Test, Salt Mass Change Results 42

Lake Albert Salinity Reduction Study - Scenario Schematisation and Testing 1

Introduction


1 Introduction

BMT WBM was commissioned by the South Australian Department of Environment, Water and

Natural Resources (DEWNR) to undertake a range of studies aimed at improving the

understanding of salinity transport and mixing mechanisms in Lake Albert.

Following a period of severe drought in the Murray Darling Basin, high rainfall through 2010 and

early 2011, resulted in significant flows in both the Darling and Murray Rivers for the first time in

over a decade. These high flows refilled the Lower Lakes and flushed considerable amounts of salt

from Lake Alexandrina. While salinity levels in Lake Albert have been significantly reduced, its

terminal nature has prevented complete flushing and salinity levels remain considerably higher

than long term pre-drought averages.

In December 2012, an investigation into options for improving Lake Albert’s water quality was

initiated by the South Australian Government. Potential management actions currently under

consideration for the reduction of salinity include:

Dredging of Narrung Narrows;

Removal or modification of the Causeway;

Connection to the Coorong;

Permanent water level structure in Narrung Narrows; and

Water level manipulations.

The aim of this investigation is to increase understanding of salinity dynamics within Lake Albert

and to provide an assessment of the proposed management options.

1.1 Summary of Associated Studies and Reports

This report is associated to a number of other reports prepared as part of the broader study for

Lake Albert. The associated studies and reports should be consulted as necessary when referred

to throughout this document. A summary of associated reports is given below.

1.1.1 Lake Albert Salinity Reduction Study - Preliminary Investigations (BMT WBM,

2013a)

This report provides details of a desktop investigation used to provide an initial assessment of a

number of potential management options aimed at improving salinity levels within Lake Albert. A

review of relevant environment characteristics of the Lower Lakes including: long-term water level

and salinity data, the stage-area-volume relationship of the Lakes, typical rates of net evaporation

and recent changes to salt mass were examined to provide a conceptual model of key factors

influencing salinity dynamics within Lake Albert. A review of previous investigations into the

hydrodynamics of the Lower Lakes and Coorong was also used to help better understand salt

dynamics in Lake Albert and provide an initial evaluation of the proposed management options.

The report contains:

A description of the environmental characteristics of Lake Albert. This includes a review of

long-term water level and salinity data sets, the relationship between lake level (stage), lake

surface area and storage volume, a summary of the rainfall and evaporation influences on the


Introduction


system and also a quantification of changes to mass of salt between April 2011 and February

2013.

A summary review of previous studies that characterise the hydrodynamics and salinity

dynamics of Lake Albert. The review focuses on extracting information that may assist in the

assessment of the five management options currently being considered to enhance salt export

from Lake Albert. Further relevant details (including figures and summary tables) from the

previous studies are presented in Appendix A of the report.

A conceptual model of the key factors that influence the salinity dynamics of Lake Albert.

Quantification of key drivers of salt mass change is provided to assist in the evaluation of the

potential management options.

A description of important features of a numerical model that would be required to accurately

quantify the five management options. The report details the benefits of model calibration and

validation as well as detailing a suggested matrix of model scenarios. These scenarios will

provide an envelope of salinity forecasts, enabling a robust assessment of likely salinity levels

in Lake Albert under a range of conditions.

A summary of key investigation outcomes and relevant conclusions and recommendations.

Further relevant details of previous reports (including figures and summary tables).

A review of the data available for future model scenarios and calibration exercises.

An initial assessment of the proposed management options indicated that:

A channel connecting Lake Albert to the Coorong capable of transferring 30 GL/month is likely

to be able to reduce salinity values within Lake Albert to below 1800 µS/cm within 6 to 12

months of operation. This option would also assist in the reduction of salinity in the Coorong

and would be less dependent on reasonable Lock 1 flows to be effective.

The report and a number of the references included in the report provide good background to the

key hydrodynamic processes and environmental drivers of the Lower Lakes and Coorong system.

1.1.2 Lake Albert Salinity Reduction Study – Model Setup and Calibration Report (BMT

WBM, 2013b)

The report details the model setup and also the achieved model calibration. The report shows that

the model is capable of simulating observed salt exchange processes from Lake Albert and

provides details of the TUFLOW-FV model software used in this study which should be referred to

as necessary.

1.1.3 Lake Albert Salinity Reduction Study – Wind Mixing Investigation

As part of a broader study into potential management options (BMT WBM, 2013a) that can reduce

salinity levels in Lake Albert, BMT WBM completed six hydrodynamic modelling simulations of the

Murray Mouth, Lower Lakes and Coorong, to simulate the exchange of salt between Lake

Alexandrina and Lake Albert due to wind mixing. Each run adopted a different wind and tide data

series corresponding to the following water years (a water year was defined as the 12 months from

1st July):

2007-2008

2008-2009

2009-2010


Introduction


2010-2011

2011-2012

2012-2013

By running the hydrodynamic model using wind and tidal conditions experienced in six different

years, the variability in expected natural (baseline) lake water mixing was assessed. The

complexity of system hydrodynamics and mixing meant that correlations between wind statistics

and salt export from Lake Albert could not be made without the use of a hydrodynamic model. By

assessing six recent years of wind data against the level of modelled salt (tracer) export and

understanding of the variability of wind mixing and exchange was possible. The results showed

that:

2008/09 winds and tides produced a low degree of salt export from Lake Albert; and

2010/11 winds and tides produced a high degree of salt export from Lake Albert.

1.1.4 Lake Albert Salinity Reduction Study – Model Scenario Schematisation and Initial

Testing

This document details the model setup for the six different scenarios and also provides results of

an initial 12 month simulation. An outline of the report is provided below in Section 1.2.

1.1.5 Lake Albert Salinity Reduction Study – Three Year Scenario Model Investigations

The report (BMT WBM, 2013c) details the evaluation of six proposed management options to

reduce Lake Albert salinity, over a broad range of environmental conditions over a three year

period.

The six management options evaluated in the study include:

a) Base-Case (i.e. do nothing)

b) Lake Cycling Option 1 (a single +/- 0.25m Lake Level variation in November/December)

c) Lake Cycling Option 2 (a single +/- 0.15m Lake Level variation in November/December)

d) Dredge Narrung Narrows and Remove Causeway

e) Coorong Connector

f) Permanent Water Level Control Structure at Narrung

The report provides a brief description of the model setup, base case and scenarios simulations. It

includes detail of all the model boundary conditions including a description of the automated

barrage approach and the models initial conditions (including 2 different initial salinity conditions). A

brief description of the six different management options is also provided.

The report also details of the model results for 12 base case simulations used to gain an

appreciation of the range of environmental conditions and outcomes that may occur. The 12

different base case simulations include a matrix of: 2 wind, 3 inflow and 3 evaporation conditions.

The report also provides details of the model results for the additional 30 scenario simulations used

to evaluate the non “base case” scenario options. The 30 different scenario simulations are for a

matrix of: 5 management options, 2 wind, 2 inflow and 2 evaporation conditions.


Introduction


1.2 Structure of Report

An outline of the remainder of the report includes:

Section 2: provides a brief description of the model setup, base case and scenarios simulations. It

includes detail of all the model boundary conditions including a description of the automated

barrage approach and the model initial conditions. A description of the six different management

options and how they were implemented/schematised by the model is also provided.

Section 3.1: provides a description of the available model results.

Section 3.2: provides details of the model results for the six scenario simulations.

Section 3.3: provides details of the model results for the additional sensitivity simulations. This

includes an assessment of an additional Quarterly Lake Cycling options, a larger Coorong

Connector and a number of different Permanent Narrung Structure options.

Section 4: provides a summary and discussion of the study findings.


Model Setup and Description of Scenario Simulations


2 Model Setup and Description of Scenario Simulations

2.1 Model Configuration

The model comprises a combination of hydrodynamics (TUFLOW-FV), waves (SWAN) and

morphology (TUFLOW-MORPH) components as described in the model calibration report (BMT

WBM, 2013b). The calibration report describes the extents, configuration and interactions between

the model components as well as how the barrages are numerically represented by the model.

The use of automated barrage logic, which allows a target Lake water level to be specified instead

of a pre-defined sequence of barrage openings, has been used in these scenario simulations. This

is an important feature of the model simulations and is further discussed in Section 2.4.8.

2.2 Model Schematisation of Six Management Options

The model was used to evaluate the influence of six potential management options on salinity

reduction in Lake Albert.

The six management options included in the study include:

a) Base-Case – This represents the “do nothing” option and assumes typical lake operations.

b) Lake Water Cycling/Manipulation – This option involves no capital expense or physical

structure but involves the deliberate manipulation (lowering and raising) of water levels to

enhance the natural export of salt from Lake Albert. In this lake cycling option there is a single

large (+/-0.2 m) deliberate change in lake levels that occurs in December through March.

c) Dredge Narrung Narrows – This option involves increasing the channel conveyance of the

narrow straight (Narrung Narrows) between Lake Alexandrina and Lake Albert. This scenario

involves significant dredging of the channel, including the removal of 5-6 million m3 of sediment

to create a channel that is a minimum 200 m wide, with an invert of -2 m AHD that runs for

approximately 12 km between the two Lakes.

d) Remove Narrung Causeway – This option involves the removal of the causeway at the ferry

crossing at Narrung.

e) Coorong Connector – This option involves constructing a channel connecting the southern

part of Lake Albert to the Coorong North Lagoon. The channel assessed in this scenario is

approximately 2 km long, 15 m wide with an invert of -1 to -1.5 m AHD and is able to convey

approximately 1000 ML/day.

f) Permanent Water Level Control Structure at Narrung – This option involves the

construction of a gated barrage structure across the Narrung Narrows so that flow between the

lakes is able to be controlled.

More details of how the different management options were represented by the model is provided

in Section 2.6.




2.3 Description of One Year Forecast Scenario Simulations

The adopted simulation period is a one year duration

nominally defined as 18th April, 2012 to 18

th April, 2013

though the use of “design” conditions mean that it is just

a one year simulation using a “design” boundary

conditions and initial conditions observed on the 18th

April, 2012. The adopted “design” boundary condition is

considered to be a fairly typical environmental condition

that should give a reasonable indication of what water

level and salt concentrations are likely to be over a

twelve month period in the Coorong and Lower Lakes if

a given management option is adopted.

2.4 Boundary Conditions for Scenario Simulations

Boundary conditions are used to “drive” model simulations. A range of “design” conditions and

historically observed datasets were used in the scenario simulations. The “design” conditions are

the same as used in BMT WBM (2012b) allowing us to compare these simulations with those

already examined in the previous study.

Boundary condition data used for the model scenario runs included:

50% AEP Lake inflows (refer Section 2.4.1);

Average “typical” (1996) direct net rainfall – evaporation (refer Section 2.4.2);

Pelican Point wind speed and direction data for 2008/09 (refer Section 2.4.3);

Offshore (Victor Harbour) water levels (tides) for 2008/09 (refer Section 2.4.4);

Offshore wave data for 2008/09 (refer Section 2.4.5);

Observed 2011/12 Salt Creek flows (refer Section 2.4.6);

Average (1996) Finniss and Currency Creek catchment inflows (refer Section 2.4.7); and

Target lake water levels and barrage opening rules (refer Section 2.4.10 & 2.4.11).

A “design” condition is often used by engineers or scientists to help evaluate and quantify a potential outcome. The “design” condition is often a synthetic dataset that has similar characteristics to observed data.




2.4.1 River Murray Inflows

Forecast flow data for Wellington was applied as an inflow just upstream of where the River Murray

flows into Lake Alexandrina. The flow forecast uses a hydrologic model (BIGMOD) to predict

inflows based on current forecast conditions and 50% AEP forecast flows (Figure 2-1). A total of

7088 GL inflow to the Lakes occurs in the 12 month base case scenario. An assumed constant

salinity of 0.25 ppt (416 μS/cm) was adopted at the boundary.

Figure 2-1 Wellington Inflow Data

Basecase Wellington Forecast Flow

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2.4.2 Net Rainfall – Evaporation

Net evaporation is a key driver of salt dynamics in the Lower Lakes and Coorong. For the scenario

simulations, typical, average monthly net evaporation rates (as supplied by DfW (now part of

DEWNR) and described in BMT WBM (2011i)) were applied to the surface of the model.

The net evaporation data used in the scenario model simulations can be seen in Figure 2-2.

Positive values represent net evaporation and indicate a removal of water from the system. Using

the South Australian Department for Water (DfW) monthly averaged time-series a total of 975 mm

of evaporative loss occurs over the year.

Figure 2-2 Lower Lakes Rainfall and Evaporation Data

Monthly Average (DfW) Lower Lakes Rainfall and Evaporation (18/4/2012 to 18/4/2013)

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Cumulative Net Evaporation Loss (m)




2.4.3 Pelican Point Wind Data

The applied wind field is another key driver of short term hydrodynamics (water levels and currents)

within the study area. The wind field creates a shear stress on the surface of the water body that

pushes the water downwind causing wind setup (and set-down). Wind driven currents and setup

can influence circulation between the Coorong’s North and South Lagoon and also between Lake

Albert and Lake Alexandrina.

Suitable wind speed and direction data for use as a model boundary is collected by DEWNR at the

Pelican Point automatic weather station (AWS). The gauge is located just to the east of

Tauwitchere Barrage between the North Lagoon and the southern part of Lake Alexandrina.

For base case scenario simulations, actual wind data recorded during between 18/4/2008 and

18/4/2009 was used as presented in Figure 2-3.

Figure 2-3 Pelican Point Wind Speed and Direction

Wind Speed and Direction (18 Apr 2008 to 18 Apr 2009)

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2.4.4 Victor Harbour Tides

A time-series of tidal water levels is required to drive the offshore water level boundary of the

hydrodynamic model. The observed Victor Harbor tidal water levels for 18/4/2008 to 18/4/2009

were used in the base case scenario simulation and are presented in Figure 2-4. During the

simulation period a number of significant tidal anomaly events were observed. A comparison of the

observed tidal anomaly to observed wind data shows a strong correlation between high wind

events and tidal anomaly.

Figure 2-4 Victor Harbour Tidal Water Level Data

Victor Harbour Tide Data (18 April 2008 to 18 April 2009)

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21

-No

v-0

8

28

-No

v-0

8

05

-Dec

-08

12

-Dec

-08

19

-Dec

-08

26

-Dec

-08

02

-Jan

-09

09

-Jan

-09

16

-Jan

-09

23

-Jan

-09

30

-Jan

-09

06

-Feb

-09

13

-Feb

-09

20

-Feb

-09

27

-Feb

-09

06

-Mar

-09

13

-Mar

-09

20

-Mar

-09

27

-Mar

-09

03

-Ap

r-0

9

10

-Ap

r-0

9

17

-Ap

r-0

9

Tid

al W

ate

r Le

vel (

m A

HD

)

-0.5

0

0.5

1

1.5

2

2.5

Tid

al A

no

mal

y (m

)

Observed Tide

Tidal Anomally (m)




2.4.5 Offshore Wave Data (BMT ARGOSS)

Modelled wave information for a location offshore of Kangaroo Island (37° S, 136° 15’ E) some

280 km south-west from the Murray Mouth was obtained from BMT ARGOSS for 1/7/2008 to

1/7/2013 at a 3-hour time interval. The BMT ARGOSS modelled wave data was extracted from a

regional WAM III wave model with a grid resolution of 1.25° (longitude) x 1.00° (latitude) driven by

wind fields from the NCEP final analysis.

These wave data were used as input to a SWAN wave model as described in BMT WBM (2013b).

The wave model calculates characteristics of the wave field near the mouth, including wave

heights, directions, periods and forces.

Wave conditions at the Murray Mouth provide a relatively minor direct contribution to overall

hydrodynamics for the Coorong and Lower Lakes. Waves have a significant role in affecting

bathymetric change at the Murray Mouth which can significantly influence hydrodynamics within the

Coorong (especially the Mouth area and North Lagoon). However, given the reasonable flows

present during the simulation period, the Murray Mouth was not at risk of significant constriction or

closure.

2.4.6 Salt Creek Inflows

Base case salt creek inflow data was provided by DEWNR and is based on observed data from

18/4/2011 to 18/4/2012 as presented in Figure 2-5. Total inflow over this period is 24.97 GL which

carries 167,085 tonnes of salt into the South Lagoon. The majority (~22 GL) of this flow occurs in

August and September while a further 3 GL occurs between mid-February to mid-March.

Figure 2-5 Salt Creek Inflow Data

Salt Creek Inflow Data

0

100

200

300

400

500

600

18/0

4/1

1

18/0

5/1

1

18/0

6/1

1

18/0

7/1

1

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8/1

1

18/0

9/1

1

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0/1

1

18/1

1/1

1

18/1

2/1

1

18/0

1/1

2

18/0

2/1

2

18/0

3/1

2

18/0

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2

Infl

ow

(M

L/D

ay)

0

5

10

15

20

25

30

Salt

Co

nc (

pp

t) o

r C

um

ula

tiv

e F

low

(G

L)

Discharge Volume (ML/Day)

Cumulative Volume (GL)

Salt Concentration (ppt)




2.4.7 Catchment Inflows

The daily time series of flow rates for Finniss and Currency Creeks were extracted from the existing

catchment (E2) model of the Murraylands region, which was developed for the EPA by BMT WBM.

Average (1996) rainfall conditions were applied to the catchment model to calculate a daily time

series of catchment inflows. The time series of inflow and salinity into the Finniss River and

Currency Creek are presented in Figure 2-6. The time-series correspond to a total annual inflow of

53.7 GL and 8.2 GL for the Finniss and Currency catchments respectively.

Due to the moderate River inflows during the simulation period, catchment inflows will have a

negligible influence on model results.

Figure 2-6 Finniss and Currency Creek Inflow Data

2.4.8 Use of Automated Barrage Opening Logic and Target Lake Water Levels

It is important to recognise the use of automated barrage / gate management logic in these

scenario simulations. Without the ability to use automated barrage operation logic, a pre-defined

sequence of gate openings would have to be developed for each model scenario. This would be

very difficult as barrage discharge is heavily influenced by actual winds, lake levels and tide

conditions. Barrage openings are typically altered on a daily to weekly basis, using lake inflow and

water level data for the preceding 3 and 7 days. This means that predicting a gate opening

sequence to maintain a target management lake water level would be extremely difficult and would

likely to have required significant model iteration, before an appropriate boundary condition was

achieved.

For the scenario simulations it may have been possible to calculate and apply a specified flow rate

at the barrages, however, such an approach does not consider the impact of downstream water

levels blocking barrage discharge and cannot simulate periods of reverse flow across the barrages.

Catchment Flow and Salinity (1996 E2 Model Data)

0

500

1000

1500

2000

2500

3000

3500

4000

4500

5000

18/0

4/1

2

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2

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2

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3

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3

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3/1

3

18/0

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3

Flo

w (

ML

/Da

y)

0.0

500.0

1000.0

1500.0

2000.0

2500.0

3000.0

3500.0

Salin

ity (

uS

/cm

)

Finniss River (ML/Day)

Currency Creek (ML/Day)

Finniss River (EC)

Currency Creek (EC)




The use of automated barrage logic (a relatively new feature in TUFLOW-FV) and a time-series of

target lake water levels allows for more realistic scenario simulations to be modelled. The

automated barrages work by comparing modelled water levels through the simulation in the Lake to

a time-series of target lake water levels (see Section 2.4.10). Every 6 hours, the model calculates

what the predicted water level in the Lake is, compares it to the time-series of target lake water

levels and calculates a water level deficit (see Equation 1). This water level deficit is then

compared to a set of rules for each of the barrages, which instruct the model how many gates at

each barrage should be opened for a given water level deficit (see Section 2.4.11).

Water Level Deficit = Predicted Model Lake Level – Target Lake Level (Equation 1)

Some examples of how the automated barrage logic works are given below:

If the water level deficit increases (i.e. the predicted model lake level starts to rise above the

target lake level), then more barrage gates are opened up, which should increase barrage

discharge and reduce the lake level back towards the target lake level.

If the water level deficit begins to decrease (i.e. the predicted model lake level starts to

approach the target lake level), then barrage gates are closed, reducing barrage discharge so

that the modelled water level begins to approach the target water level.

If the modelled lake level is below the target water level, then eventually all the barrage gates

will be closed so that the lake level should rise until the target lake level is achieved.

2.4.9 Adopted Model Barrage Structures

Table 2-1 summarises the adopted hydraulic properties used in the model. A description of the

numerical implementation of the barrage discharge calculations are provided in BMT WBM

(2013b).

The management target water levels used in the scenario simulations are presented in Section

2.4.10, while the adopted barrage operation rules are presented in Section 2.4.11.

Table 2-1 Adopted Hydraulic Properties for Barrages

Barrage Full Opening Width Sill Level

Goolwa 458.4m (128 gates) -0.45 m AHD (two logs removed)

Mundoo 90m (26 gates) -1.0 m AHD

Boundary Creek 21.5m (6 gates) -1.0 m AHD

Ewe Island 431.35m (121 gates) -0.05 m AHD

Tauwitchere 1251.3m (322 gates) -0.05 m AHD

2.4.10 Target Management Lake Water Levels (Barrage Openings)

A time-series of target management lake water levels is required for use with the automated

barrage opening logic, which was adopted for use in the scenario simulations. The time-series of

target water levels was provided by DEWNR for use in BMT WBM (2012b) and is presented in




Figure 2-7. It was used in conjunction with barrage operating rules (see Section 2.4.11) to

determine actual barrage gate openings used by the model.

Figure 2-7 Standard and Cycled Target Lake Water Levels

2.4.11 Adopted Barrage Operating Rules

The use of the automated barrage logic requires a target water level (as defined above) and a set

of gate operational rules that relate a water level deficit to a number of gates that should be open.

The gate opening logic as defined in Table 2-2 was adopted for the base case scenario simulation.

The table defines the proportion of total gates that should be opened for a given water level deficit

(see definition provided in Section 2.4.8). These gate opening rules were used for all scenario

simulations.

Table 2-2 Adopted Gate Opening Rules for Barrages

Proportion of Total Structure Length Open

WL Deficit (m)

Goolwa Mundoo Boundary

Creek Ewe Island Tauwitchere

-0.05 0 0 0 0 0

-0.02 0.05 0.05 0 0.05 0.05

0.00 0.1 0.1 0 0.1 0.1

0.01 0.2 0.2 0.167 0.2 0.2

0.05 0.5 0.5 0.167 0.55 0.6

0.10 0.7 1 0.167 0.55 0.6

0.20 1 1 1 0.8 0.8

0.30 1 1 1 1 1

Standard and Cycled Target Lake Level

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

18/0

4/1

2

18/0

5/1

2

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6/1

2

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7/1

2

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8/1

2

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9/1

2

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0/1

2

18/1

1/1

2

18/1

2/1

2

18/0

1/1

3

18/0

2/1

3

18/0

3/1

3

18/0

4/1

3

Ta

rge

t W

ate

r L

ev

el (m

AH

D)

Cycled Lake Level Target

Standard Lake Level Target




2.5 Model Initial Conditions

Initial conditions required by the model include:

Water level;

Salinity; and

Murray Mouth bathymetry.

Details and assumptions for the derivation of initial conditions used for model scenarios are

provided in the following sections.

2.5.1 Initial Water Levels

The initial water levels adopted in the base case scenario are based on an examination of DEWNR

water level data for the 18th April 2012, as presented on the River Murray Data website, which

provides water levels and salinity data in the Lower Lakes and Coorong. A degree of engineering

judgement was applied to determine a representative water level for each water body as presented

in Table 2-3.

Table 2-3 Initial Water Levels (18th

April, 2012)

Water Body

Level (m AHD)

Lake Alexandrina 0.65

Lake Albert 0.68

North Lagoon 0.52

South Lagoon 0.51

2.5.2 Initial Salinity and Salt Mass

The initial salinity adopted for the base case and scenario simulations was based on:

Salinity transect data collected in the Coorong on the 18th April, 2012;

Salinity transect data collected in Lake Alexandrina and Albert on the 13th April, 2012; and

DEWNR salinity data as presented on the River Murray Data website.

The model transect data (as presented in Figure 2-8) was used to generate a continuous digital

surface of salinity using an inverse distance weighted (IDW) extrapolation method. In areas where

no transect data was available, assumed salinities (based on DEWNR gauge data) were used to

complete the initial conditions for the model. A map of the initial model salinity for the start of the

simulation period (18th April, 2012) is presented in Figure 2-8.




Table 2-4 Initial Salinity Assumptions

Water Body

Salt Concentration (ppt)

Goolwa Channel 0.3

Murray Mouth Area 1.0

Offshore 35

End of South Lagoon 90 - 102

The initial mass of salt at the start of the simulation (18th April, 2012) is provided in Table 2-5. The

mass was calculated by multiplying the concentration of each cell by the average cell depth and

integrating the mass over each region. It shows that a significant mass of salt still resides in the

South Lagoon.

Table 2-5 Approximate Initial Salt Mass (18th

April, 2012)

Area

Salt Mass (Tonnes)

Lake Alexandrina 459,160

Lake Albert 757,820

Coorong (Total – Combined North and South Lagoon) 15,487,780

North Lagoon 1,189,710

South Lagoon 14,298,070

2.5.3 Murray Mouth Bathymetry

The model’s initial Murray Mouth bathymetry for the base case and scenario runs are based on a

bathymetric survey data collected by SA Water on the 3rd

of April, 2012. As the bathymetry data

does not extend offshore beyond the mouth, these data were combined with model predictions of

offshore bathymetry taken from the latest model calibration run (see BMT WBM, 2012a), which

provide indicative bathymetry for the 1st November 2011. A map of starting bed levels (for 18

th

April, 2012) for the Murray Mouth area is presented in Figure 2-9.




Figure 2-8 Initial Model Salinity (ppt) and Transect Locations




Figure 2-9 Initial Murray Mouth Model Bathymetry, 18th

April 2012

2.11




2.6 Model Scenario Schematisation for Initial Assessment

Six different management options were schematised (represented by the model) including:

a) Base-Case – This represents the “do nothing” option and assumes typical lake operations.

b) Lake Water Cycling/Manipulation – This was represented in the model by applying a different

target lake level time-series (see Figure 2-7). This time-series used in conjunction with the

automated barrage logic (refer Section 2.4.8, 2.4.10 & 2.4.11) which would then close or open

barrages so that the target lake level is realised in the model.

c) Dredge Narrung Narrows – This option involves increasing the channel conveyance of the

narrow straight (Narrung Narrows) between Lake Alexandrina and Lake Albert. This scenario

was represented in the model by applying a different (typically deeper) bathymetry along

Narrung Narrows as presented in Section 2.6.2.

d) Remove Narrung Causeway – This option involves the removal of the causeway at the ferry

crossing at Narrung, it was implemented in the model by altering the bathymetry in the vicinity

of the Causeway.

e) Coorong Connector – This option involves constructing a channel connecting the southern

part of Lake Albert to the Coorong North Lagoon. It was implemented in the model by altering

the mesh to allow a connection between Lake Albert and the Coorong’s North Lagoon. A

barrage control structure was also specified to simulate a gate structure that could prevent

backflow from the Coorong in Lake Albert. Channel and control structure details are presented

in Section 2.6.3.

f) Permanent Water Level Control Structure at Narrung – This option involves the

construction of a gated barrage structure at Narrung Narrows so that flow between the lakes is

able to be controlled. It was implemented in the model using a range of nodestring structure

elements as described in Section 2.6.4.

The scenarios were defined based on information provided in an email from DEWNR on 5th July,

2013 and include advice from SKM.

2.6.1 Model Mesh Development for Scenarios

The modular setup of TUFLOW-FV inputs enabled a single model mesh (see Figure 2-10) to be

used for all model scenarios. Any of the six management scenarios can be developed with only

minor changes to some of the model input files. The use of a single mesh also facilitates the

comparison of results within SMS or EXCEL and only a single set of model initial conditions and

boundary conditions was required.

The means that even though the mesh has elements that are used to represent the Coorong

Connector channels (in this case to alternative locations have been included in the base mesh), the

use of separate cell centre mesh elevation files can be used to specify which (or neither) channel is

open as required.




Figure 2-10 TUFLOW-FV Management Scenario Mesh




2.6.2 Details of Narrung Narrows Dredging Scenario

This option involves increasing the channel conveyance of the narrow straight (Narrung Narrows)

between Lake Alexandrina and Lake Albert. This scenario involves significant dredging of the

channel, including the removal of 5-6 million m3 of sediment to create a channel that is a minimum

200 metres wide, with an invert of -2 m AHD that runs for approximately 12 km between the two

Lakes. The existing bathymetry of the Narrung Narrows adopted in the Base Case (and other)

simulation is presented in Figure 2-11, while the changed bathymetric data used in the Dredging

scenario is presented in Figure 2-12.

2.6.3 Details of Coorong Connector Scenario

This option investigates the benefits of constructing a channel between the southern part of Lake to

the Coorong North Lagoon. The channel assessed in this scenario is approximately 2 km long,

15 metres wide with an invert of -1 (Lake Albert) to -1.5 m AHD (North Lagoon) and is able to

convey approximately 1000 ML/day. The adopted channel location is from Kennedy Bay in Lake

Albert to Noonameena in the North Lagoon (see Figure 2-13).

A barrage control structure located midway along the channel was specified to simulate a gate

structure that could prevent backflow from the Coorong in Lake Albert. The gate structure uses a

nodestring, automated weir structure object (similar to that used for the Coorong Barrages (see

Sections 2.4.8) with a -1.25 m AHD sill. The structure has been implemented to automatically

(instantaneously) close to prevent reverse flow (i.e. flow from the Coorong into Lake Albert)

occurring. The gate control is also implemented to stop lake levels falling significantly (currently set

to -0.011 m) below target lake levels (as defined in Section 2.4.10).

Details of the achieved channel discharge are presented in Section 3.2.5.




Figure 2-11 Base Case Mesh Narrung Narrows Bathymetry




Figure 2-12 Dredged Scenario Mesh Narrung Narrows Bathymetry




Figure 2-13 Schematisation of Coorong Connector




2.6.4 Details of Permanent Narrung Control Structure

Advice from DEWNR/SKM indicated that the location of the permanent Narrung Control Structure

would be an approximate 230 metre opening that is the route of the current car ferry service at

Narrung. The control structure was considered to be similar to the existing automated hydraulic

barrages at Tauwitchere with a sill level of between -2 to -1.5 m AHD. The proposed structure was

implemented in the model using three nodestring structures including:

1) The existing causeway to the east of the channel;

2) A short ramp to the west of the channel; and

3) The 230 m long gated control structure with a -2 m AHD sill and a time series (see Figure 2-15

and Figure 2-16) specifying when the gates were open or closed.

The derivation of the gate opening, closure sequence is described in Section 3.3.3.

Figure 2-14 Proposed Location of Permanent Narrung Control Structure (Source: SKM)




Figure 2-15 Narrung Control Structure One Week Gate Closure Sequence

Figure 2-16 Narrung Control Structure Two Week Gate Closure Sequence

Lake Levels and Narrung Gate Closures

0.40

0.50

0.60

0.70

0.80

0.90

1.00

18/0

4/2

012

16/0

5/2

012

13/0

6/2

012

11/0

7/2

012

8/0

8/2

012

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012

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012

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012

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012

26/1

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012

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013

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013

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013

17/0

4/2

013

Hin

dcast

Lake W

ate

r L

evel (m

AH

D)

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

Str

uctu

re S

ill L

evel (m

AH

D)

Lake Alexandrina (Water Level)

Lake Albert (Water Level)

1 Week Gate Closure Timing Sequence

Gates Closed

Gates Open

Lake Levels and Narrung Gate Closures

0.40

0.50

0.60

0.70

0.80

0.90

1.00

18/0

4/2

012

16/0

5/2

012

13/0

6/2

012

11/0

7/2

012

8/0

8/2

012

5/0

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012

3/1

0/2

012

31/1

0/2

012

28/1

1/2

012

26/1

2/2

012

23/0

1/2

013

20/0

2/2

013

20/0

3/2

013

17/0

4/2

013

Hin

dcast

Lake W

ate

r L

evel (m

AH

D)

-2.5

-2.0

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

Str

uctu

re S

ill L

evel (m

AH

D)

Lake Alexandrina (Water Level)

Lake Albert (Water Level)

2 Week Gate Closure Timing Sequence

Gates Closed

Gates Open


Model Output and Results


3 Model Output and Results

3.1 Description of Model Output

3.1.1 Water Level Output

Representative water levels for each of the four water bodies in the study region predicted by the

model are presented below. The locations adopted for each of the water bodies are summarised in

Table 3-1 and presented in Figure 3-1. By selecting locations near the centre of each of the water

bodies the influence of wind setup is minimised. It is important to note that wind setup could create

water level differences across the lakes of up to ±0.2 m in very strong (>10 m/s) winds.

Table 3-1 Summary of Output Locations

Water Body Location

Lake Alexandrina Centre (see Figure 3-1)

Lake Albert Centre (see Figure 3-1)

Coorong – North Lagoon Long Point (see Figure 3-1)

Coorong – South Lagoon Woods Well (see Figure 3-1)

3.1.2 Salinity Outputs

Salinity results for the four key water bodies in the Coorong and Lower Lakes system are

presented at the same output locations summarised above. The locations are broadly

representative for salinity in each of the water bodies, although the presence of high longitudinal

salinity gradients in the North Lagoon should be noted and results at the chosen location should be

interpreted with caution. The locations adopted are fairly representative for each of the water

bodies are summarised in Table 3-1 and displayed in Figure 3-1.

3.1.3 Water Volume and Salt Mass Change

TUFLOW-FV is capable of tracking the volume and mass (salt) flux between any cell in the model

domain. This allows the presentation of flow (water movement) data and also the calculation of the

total volume or salt mass change with time to be calculated.




Figure 3-1 Map of Model Output Locations




3.2 Comparison of Management Option Results

The following sections describe the results of the six different lake configurations or management

options, which include:

a) Base Case (i.e. do nothing);

b) Remove Causeway;

c) Dredge Narrung Narrows;

d) Permanent Water Level Control Structure at Narrung;

e) Summer Lake Water Level Cycling; and

f) Coorong Connector.

The results presented in this Section for the Permanent Water Level Control Structure at Narrung

Scenario adopt the -2 m AHD and always open sill configuration, which (as discussed in Section

3.3.3) is the most effective configuration for that scenario.

3.2.1 Water Levels

Modelled water levels for the six proposed management scenarios are presented in Figure 3-2

(Lake Alexandrina), Figure 3-3 (Lake Albert), Figure 3-4 (the North Lagoon (Long Point)) and

Figure 3-5 (South Lagoon (Woods Well)).

Overall, modelled water levels in Lake Alexandrina (Figure 3-2) for the six scenarios closely match

the target lake level, though during periods of low flow, the current barrage rules allow lake level to

fall up to 10 cm below the target levels. Also, the influence of storm surges blocking barrage

discharge and increasing lake levels can be observed in mid-May, 2012 and early-March, 2013.

Lake levels for the five management options that use the same “standard” lake level target are

nearly the same, while the influence of the summer lake cycle option is obvious.

In Lake Albert (Figure 3-3) modelled water levels are similar to those in Lake Alexandrina;

however, the influence of wind seiche results in a greater water level fluctuation (up to 10 to

15 cm), and the differences in water levels between the management scenarios is more apparent.

Water levels in Lake Albert for the Coorong Connector option are 1 to 2 cm below the base case

water levels as slightly more water is transported into the Coorong than in the other options. The

impact on water levels due to increase conveyance at Narrung for the Dredging scenario is

apparent, and the influence of wind on water levels for the summer lake cycle can also be

observed in the water level signal.

Water levels in the North Lagoon (Figure 3-4 ) and South Lagoon (Figure 3-5) are primarily driven

by tides, winds and barrage discharge. The influence of the Coorong Connector option in raising

water levels in the Coorong is marginally apparent, likewise the changes to barrage discharge

required to implement the summer lake cycle option also influences Coorong water levels.

3.2.2 Salinity (Salt Concentration)

Modelled salinity levels for the six scenarios are presented in Figure 3-6 (Lake Albert), Figure 3-7

(Lake Alexandrina), Figure 3-8 (the North Lagoon (Long Point)) and Figure 3-9 (South Lagoon

(Woods Well)).

The effectiveness of the six potential management options for salinity reduction in Lake Albert is

presented in Figure 3-6 and summarised in Table 3-2. The results shows that the Coorong




Connector options is by far the most effective at reducing salinity in Lake Albert, with salinity levels

dropping by approximately 60% from 4935 μS/cm to below 2000 μS/cm over a 12 month period.

This compares favourably to the Base Case in which there is only an 8.1% reduction in salinity over

the same simulation period. The Summer Lake Cycle scenario is the only other management

option that results in a significant salinity reduction compared to the Base Case, with the single

water level fluctuation being able to reduce salinity by 64% above the Base Case scenario.

The causeway removal option has minimal impact on lake system hydrodynamics or the salt

dynamics of Lake Albert, i.e. it reduces the change to salt concentration relative to the Base Case

(i.e. “Do Nothing”) scenario by less than 3%. A somewhat surprising outcome of the study is that

dredging Narrung Narrows would be less effective in terms of salinity reduction in Lake Albert than

the Base Case. Further discussion of the processes responsible for that result is presented in

Section 3.2.4. The construction of a permanent gate structure with a -2 m AHD sill would reduce

the salinity reduction potential of the Lake Albert by 25% compared to the Base Case. Any closure

of the structure’s gates would further reduce the ability for salt to be exported (refer Section 3.3).

Table 3-2 Summary of Lake Albert Salinity Results

Scenario Final Salinity (μS/cm)

Salinity Reduction

(μS/cm)

Salinity Reduction

(%)

% Comparison to Base Case

Base Case 4533.1 401.7 8.1 0.0

Remove Causeway 4522.7 412.1 8.4 2.6

Dredge Narrung 4575.2 359.6 7.3 -10.5

Permanent Structure (-2 m AHD Sill, Always Open)

4634.6 300.2 6.1 -25.3

Summer Lake Cycle 4274.8 660.0 13.4 64.3

Coorong Connector 1988.6 2946.2 59.7 633.4

There are minor differences in predicted salinity in Lake Alexandrina (Figure 3-7) between the six

scenarios; with the Coorong Connector achieving a moderate reduction in Lake Alexandrina salinity

as saltier water from Lake Albert is transported into the Coorong instead of mixing into Lake

Alexandrina. The impact of a Summer Lake Water Level Cycle is also evident with salinity

increasing in Lake Alexandrina as lake levels fall and saltier water from Lake Albert is transported

into Lake Alexandrina.

The model predicts minor differences in salinity levels in the North Lagoon (Figure 3-8) and South

Lagoon (Figure 3-9) for four of the six scenarios. For the Coorong Connector scenario, there is a

significant reduction in peak salinity levels in the North Lagoon due to dilution and also a change to

the water balance of the Coorong. In the South Lagoon, the impact of the Coorong Connector in

reducing salinity is also evident as the reduced salt load results in a considerable drop in South

Lagoon salinity by the end of the 12 month simulation. The Summer Lake Level scenario also

changes salinity levels in the Coorong, due to changes in barrage discharge. That option can

influence water levels and hence the salinity gradient along the Coorong, however, it will only have

a minor impact on the overall salt load to the Coorong.




Figure 3-2 Modelled Water Levels in Lake Alexandrina for Proposed Scenarios

Figure 3-3 Modelled Water Levels in Lake Albert for Proposed Scenarios

Lake Alexandrina Modelled Water Levels

0.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.9018-A

pr-

12

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ay-1

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ul-12

08-A

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17-A

pr-

13

Wate

r L

evel (m

AH

D)

Summer Lake Level Cycle

Permanent Narrung Structure (-2mAHD Sill - Always Open)

Dredge Narrung

Coorong Connector

Remove Causeway

Base Case

Lake Albert Modelled Water Levels

0.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

18-A

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ar-

13

17-A

pr-

13

Wate

r L

evel (m

AH

D)



Dredge Narrung

Coorong Connector

Remove Causeway

Base Case




Figure 3-4 Modelled Water Levels in the North Lagoon for Proposed Scenarios

Figure 3-5 Modelled Water Levels in the South Lagoon for Proposed Scenarios

North Lagoon (Coorong) Modelled Water Levels

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

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pr-

13

Wa

ter

Le

ve

l (m

AH

D)



Dredge Narrung

Coorong Connector

Remove Causeway

Base Case

South Lagoon (Coorong) Modelled Water Levels

-0.2

-0.1

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

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13

17-A

pr-

13

Wa

ter

Le

ve

l (m

AH

D)



Dredge Narrung

Coorong Connector

Remove Causeway

Base Case




Figure 3-6 Modelled Salinity in Lake Albert for Proposed Scenarios

Figure 3-7 Modelled Salinity in Lake Alexandrina for Proposed Scenarios

Lake Albert Modelled Salinity

0

1,000

2,000

3,000

4,000

5,000

6,0001

8-A

pr-

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ay-1

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ar-

13

17-A

pr-

13

Sa

lin

ity

(E

C)



Dredge Narrung

Coorong Connector

Remove Causeway

Base Case

Lake Alexandrina Modelled Salinity

0

100

200

300

400

500

600

700

18-A

pr-

12

16-M

ay-1

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13

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pr-

13

Sa

lin

ity

(E

C)



Dredge Narrung

Coorong Connector

Remove Causeway

Base Case




Figure 3-8 Modelled Salinity in the North Lagoon for Proposed Scenarios

Figure 3-9 Modelled Salinity in the South Lagoon for Proposed Scenarios

North Lagoon (Coorong) Modelled Salinity

0

10

20

30

40

50

60

18-A

pr-

12

16-M

ay-1

2

13-J

un-1

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11-J

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2

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05-S

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an-1

3

20-F

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3

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ar-

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17-A

pr-

13

Sa

lin

ity

(p

pt)



Dredge Narrung

Coorong Connector

Remove Causeway

Base Case

South Lagoon (Coorong) Modelled Salinity

0

20

40

60

80

100

120

140

18-A

pr-

12

16-M

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(p

pt)



Dredge Narrung

Coorong Connector

Remove Causeway

Base Case




3.2.3 Cumulative Salt Mass Change

Modelled Lake Albert salt mass change is presented in Figure 3-10 (expressed in Tonnes of salt),

Figure 3-11 (expressed in percentage change from initial salt mass) and summarised in Table 3-3

for the six scenarios. The percentage salt mass reduction is based on an initial Lake Albert salt

mass of 757,820 tonnes (see Section 2.5.2.).

Table 3-3 Summary of Lake Albert Salt Mass Change Results

Scenario Salt Mass

Reduction (tonnes)

Salt Mass Reduction

(%) % Comparison to Base Case

Base Case 98,678 13.0 0.0

Remove Causeway 100,213 13.2 1.6

Dredge Narrung 91,546 12.1 -7.2


84,324 11.1 -14.5

Summer Lake Cycle 136,882 18.1 38.7

Coorong Connector 465,655 61.4 371.9

The results show that in the Base Case (i.e. do nothing) scenario 98,678 tonnes of salt would be

transported from Lake Albert during the 12 month simulation period (a decrease in salt mass of

13.0%). The decrease to the salt mass is higher than the change in concentration as the water

level (and hence lake volume) has fallen. Since the salt concentration is reported in the centre of

Lake Albert it does not account for the possibility that fresher water may be present in the north-

western segment of the lake.

A comparison of the effectiveness of the six management options in reducing salt mass is similar to

that for reducing salt concentration (see Section 3.2.2). The results show that the Coorong

Connector option is able to remove the most salt from Lake Albert, transporting a net salt mass of

465,655 tonnes from the lake. This includes 590,792 tonnes of salt transported into the Coorong

through the connector channel and 125,137 tonnes of (lower concentration) salt flows from Lake

Alexandrina into Lake Albert, to balance the flow out of the connector channel (see Section 3.2.5.).

This Coorong Connector is able to reduce the Lake Albert salt mass by 61% which is nearly

4 times as much as the Base Case scenario.

The Summer Lake Cycle scenario is the next most efficient option removing 40% more salt than

the Base Case in a single lake cycle event. All other scenarios behave in a similar manner to the

Base Case, with the exception of the Remove Causeway option which removes marginally (2%)

more salt. The Dredge Narrung scenario actually removes 7% less salt than the Base Case

scenario, while the Permanent Structure with a -2 m AHD sill with gates open removes

approximatley15% less salt than the Base Case scenario.

An examination of the timing of the salt mass change (Figure 3-10) shows that there is a small

increase in salt mass in Lake Albert at the start of the simulation as the lake level is increased over

an approximate 2 month period (see Figure 2-7). From June to October the lake levels are held

relatively constant and wind driven transport and mixing is able to remove nearly 100,000 tonnes of

salt from Lake Albert. As evaporation increases in Spring and Summer, the transport of salt into

Lake Albert to satisfy evaporation demand is close to the wind driven mixing of salt out of Lake

Albert (which is slightly assisted by a lowering of Lake water levels through Summer), with minimal

salt mass change occurring in December through April.

For the Summer Lake Cycle option, the salt mass change is the same as the Base Case until

December. During December, water levels are deliberately lowered, and additional salt is exported




from Lake Albert to Lake Alexandrina. As the water level is raised again during February, there is

an increase of salt mass which is marginally greater than the Base Case amount; however, as the

water level falls again in March, additional salt is removed from Lake Albert.

3.2.4 Narrung Flows and Cumulative Volume Change

Cumulative Lake Albert volume change presented in Figure 3-12 and shows that the overall water

balance is driven by evaporation and lake level change, and hence there is insignificant difference

between the six management options, though the influence of the lake cycle option on lake volume

is evident. Evaporative demand in Lake Albert accounts for a volume of 170 GL (assuming a loss

of 0.975 m and surface area of approximately 175 km2), while each 1 cm change in water level in

Lake Albert equates to a volume change of -1.7 GL. Over the 1 year simulation, the net change in

Lake Albert water level is about 8cm (a decrease from 0.68 m AHD to 0.60 m AHD) which is a

volume change of -14 GL. Adding these volume changes together results in a net volume change

of 156 GL which agrees with the cumulative Lake Albert volume change presented in Figure 3-12.

While there is insignificant difference between the six options in terms of long-term cumulative

volume change in Lake Albert, there is a difference in instantaneous flows at Narrung in a number

of the options as presented in Figure 3-13 and Figure 3-14 (6 weeks only). The graphs show

positive flows as those entering Lake Albert, while negative flows are discharges from the Lake.

Comparison of the Narrung flow data to wind data (Figure 2-3 and Figure 3-15 (6 weeks only))

reveals that the majority of flow events are due to wind setup. An examination of Figure 3-14 and

Figure 3-15 shows that in the week following the 9th May, conditions are mostly calm with a south

to south-easterly sea-breeze forming during the day, and then dropping in the evening. This

produces a day time flow of water out of Lake Albert, which then flows back into Lake Albert as the

wind speed drops and the water levels in the lake return to an equilibrium (i.e. flat) position.

The results show that dredging at Narrung increases the channel conveyance (and hence

magnitude of peak flow exchanges) but as shown in Figure 3-12 it does not alter the overall water

balance, though in summer it does allow the evaporative demand to be more quickly met (and

hence may allow salt to penetrate further into Lake Albert). The flow results also show that there is

slightly higher inflow in the Coorong Connector option than is required to satisfy the discharge (as

quantified in Section 3.2.5) through the Coorong Connector channel. Further examination of Figure

3-14 shows that there is no difference in flows at Narrung between the Base Case and Remove

Causeway scenario; however, the Permanent Structure scenario slightly reduces peak flows for a

number of wind events.

3.2.5 Coorong Connector Flow Summary

For the Coorong Connector simulations, the total discharge from Lake Albert into the Coorong for

the 1 year simulation period is 315 GL. The discharge represents an average flow of 862 ML/day,

however, actual daily discharge varies seasonally (mostly due to difference in lake water levels)

with predicted daily flow varying from between ~500 ML/day and ~1100 ML/day as presented in

Figure 3-16. It is assumed that the channel gates would automatically close if reverse flow is about

to occur (i.e. when the Coorong water level is higher than Lake Albert water level), which is why the

flow is occasionally 0 ML/day.




Figure 3-10 Modelled Lake Albert Salt Mass Change (Tonnes) for Six Scenarios

Figure 3-11 Modelled Lake Albert Percentage Salt Mass Change for Six Scenarios

Lake Albert Cumulative Salt Mass Change - 1 Year Scenario Simulations

-500,000

-450,000

-400,000

-350,000

-300,000

-250,000

-200,000

-150,000

-100,000

-50,000

0

50,000

18-A

pr

16-M

ay

13-J

un

11-J

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08-A

ug

05-S

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03-O

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31-O

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28-N

ov

26-D

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23-J

an

20-F

eb

20-M

ar

17-A

pr

Cu

mm

ula

tiv

e S

alt

Flu

x (

To

nn

es)


Permanent Narrung Structure (-2m AHD Sill - Always Open)

Dredge Narrung

Coorong Connector

Remove Causeway

Base Case

Lake Albert Cumulative Salt Change (%) - 1 Year Scenario Simulations

-70

-60

-50

-40

-30

-20

-10

0

10

18-A

pr

16-M

ay

13-J

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11-J

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08-A

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26-D

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20-M

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17-A

pr

Cu

mm

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tiv

e T

race

r F

lux

(%

of

Init

ial S

alt

Ma

ss

) .



Dredge Narrung

Coorong Connector

Remove Causeway

Base Case




Figure 3-12 Modelled Lake Albert Volume Change for Six Scenarios

Figure 3-13 Modelled Narrung Flows for Six Scenarios

Lake Albert Cumulative Volume Change - 1 Year Scenario Simulations

0

20

40

60

80

100

120

140

160

180

200

18-A

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(G

L)



Dredge Narrung

Remove Causeway

Base Case

Coorong Connector (Volume Change)

Modelled Flow Timeseries at Narrung - 1 Year Scenario Simulations

-40,000

-30,000

-20,000

-10,000

0

10,000

20,000

30,000

40,000

18-A

pr

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ay

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Ins

tan

tan

eo

us

Flo

w (

ML

/Da

y)



Dredge Narrung

Coorong Connector

Remove Causeway

Base Case

Coorong Connector (Channel)




Figure 3-14 Modelled Narrung Flows for Six Scenarios (six weeks)

Figure 3-15 Applied Wind Data (six weeks)

Modelled Flow Timeseries at Narrung - 1 Year Scenario Simulations (18/4 - 30/5/2012)

-20,000

-15,000

-10,000

-5,000

0

5,000

10,000

15,000

20,0001

8-A

pr

25-A

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02-M

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09-M

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16-M

ay

23-M

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30-M

ay

Ins

tan

tan

eo

us

Flo

w (

ML

/Da

y)



Dredge Narrung

Coorong Connector

Remove Causeway

Base Case

Coorong Connector (Channel)

Wind Speed and Direction (18 Apr 2008 to 30 May 2008)

0.0

1.0

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

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-May

-08

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-May

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-08

Win

d S

pe

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(m

/s)

-540

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-270

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-90

0

90

180

270

360

Win

d D

ire

ctio

n (

TN)




Figure 3-16 Coorong Connector Channel Modelled Flow Time series

3.3 Sensitivity Testing of Scenarios

The following sections describe the results of a number of sensitivity tests undertaken to provide

further information regarding a number of the management scenarios.

3.3.1 Coorong Connector Channel Conveyance

The performance of a larger channel (with increased channel conveyance) connecting Lake Albert

to the Coorong was also assessed. The option is nearly identical to the Coorong Connector

channel previously described although it uses a larger channel/gate, capable of higher discharge.

The total flow predicted for a larger Coorong Connector Channel for the year was 461 GL

compared with the 315 GL for the smaller channel. The discharge represents an average flow of

1263 ML/day compared to 862 ML/Day which is close to a 50% increase in discharge. Again actual

daily discharge varies seasonally (mostly due to difference in lake water levels) with actual daily

flow varying from between approximately 600 ML/day and approximatley1350 ML/day.

The results of the simulation of the larger channel are compared to the previous Coorong

Connector and Base Case results in Table 3-4 (salinity), Table 3-5 (salt mass change) and Figure

3-17 (salinity time-series). The results show that the increased discharge further assists with the

export of salt (an additional 80,000 tonnes) resulting in a final Lake Albert of salinity of 1410 μS/cm.

Modelled Flow for Coorong Connector Channel

0

200

400

600

800

1,000

1,200

1,400

18-A

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11-J

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26-D

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23-J

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w (

ML

/Da

y)




3.3.2 Quarterly Lake Cycling Performance

The performance of a quarterly lake level cycle option with lake levels fluctuating between 0.6 and

0.8 m AHD every three months (see Figure 3-18) was also assessed. While insufficient lake inflows

or wind conditions result in target levels not always being achieved (see Figure 3-19), the results

of the simulation as presented in Table 3-4 (salinity), Table 3-5 (salt mass change) and Figure 3-17

(salinity time-series) show that the additional water level fluctuations enhance salt export. The

results show that the increased frequency of discharge is able to export an additional

42,000 tonnes (~30% more) of salt compared to the summer cycled option. This option is predicted

to reduce salinity in Lake Albert from about 5000 μS/cm to below 4000 μS/cm within 12 month

which is an almost 140% increase in salinity reduction compared to the Base Case scenario.

3.3.3 Permanent Narrung Structure Options

In order to better understand the performance of the Permanent Narrung Structure, a number of

additional model simulations were required including: tests on the adopted structure sill level (-2, 1

or 0 m AHD), and also the duration of gate closures (either one or two weeks). The tests on the

different sill level assumed that the structure was always open, while for the gate closure option a

sequence of 12 closure events were selected (see Figure 2-15 and Figure 2-16). This involved

selecting significant wind events that produced over a 10 cm water level difference between Lake

Albert and Lake Alexandrina and the closing the structure for a period of either one or two weeks to

prevent the immediate levelling of lake levels and potentially enhance mixing and hence salt

export.

The results of the simulations of the five simulations are presented in Table 3-4 (salinity), Table 3-5

(salt mass change) and Figure 3-17 (salinity time-series). The results show that reducing the sill

level of the gate on the structure reduces the level of salt export, and that any closures of the gate

reduce the potential of export of salt from Lake Albert.

Table 3-4 Summary of Lake Albert Sensitivity Test, Salinity Results

Scenario Final Salinity (μS/cm)

Salinity (μS/cm) Reduction

% Salinity Reduction


Base Case 4533.1 401.7 8.1 0.0

Summer Lake Cycle 4274.8 660.0 13.4 64.3

Quarterly Lake Cycle 3975.8 959.0 19.4 138.7

Coorong Connector 1988.6 2946.2 59.7 633.4

Coorong Connector (Larger Channel)

1410.1 3524.8 71.4 777.4

Permanent Structure (-2mAHD Sill, Always Open)

4634.6 300.2 6.1 -25.3


4645.8 289.0 5.9 -28.1

Permanent Structure (0mAHD Sill, Always Open)

4920.2 14.6 0.3 -96.4

Permanent Structure (-2mAHD Sill, 1 week closures)

4766.9 167.9 3.4 -58.2


5034.0 -99.2 -2.0 -124.7




Table 3-5 Summary of Lake Albert Sensitivity Test, Salt Mass Change Results

Scenario Salt Mass Reduction (tonnes)

% Salt Mass Reduction


Base Case 98,678 13.0 0.0

Summer Lake Cycle 136,882 18.1 38.7

Quarterly Lake Cycle 178,866 23.6 81.3

Coorong Connector 465,655 61.4 371.9


545,999 72.0 453.3


84,324 11.1 -14.5


82,692 10.9 -16.2

Permanent Structure (0mAHD Sill, Always Open)

44,697 5.9 -54.7


65,509 8.6 -33.6


28,047 3.7 -71.6

Figure 3-17 Modelled Lake Albert Salt Mass Change (Tonnes) Sensitivity Test Results

Lake Albert Cumulative Salt Mass Change - 1 Year Scenario Sensitivity Tests

-600,000

-550,000

-500,000

-450,000

-400,000

-350,000

-300,000

-250,000

-200,000

-150,000

-100,000

-50,000

0

50,000

18-A

pr

16-M

ay

13-J

un

11-J

ul

08-A

ug

05-S

ep

03-O

ct

31-O

ct

28-N

ov

26-D

ec

23-J

an

20-F

eb

20-M

ar

17-A

pr

Cu

mm

ula

tiv

e S

alt

Flu

x (

To

nn

es)

Permanent Narrung Structure (-2m AHD Sill - 2 Week Closures)

Permanent Narrung Structure (-2m AHD Sill - 1 Week Closures)



Quarterly Lake Level Cycle

Coorong Connector


Base Case




Figure 3-18 Standard and Cycled Target Lake Water Levels

Figure 3-19 Target and Modelled Lake Water Levels (Quarterly Cycling)

Standard and Cycled Target Lake Level

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

18/0

4/2

012

18/0

5/2

012

18/0

6/2

012

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012

18/0

8/2

012

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012

18/1

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012

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012

18/1

2/2

012

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1/2

013

18/0

2/2

013

18/0

3/2

013

18/0

4/2

013

Targ

et

Wate

r L

evel (m

AH

D)

Quarterly Cycled Lake Level Target

Summer Cycled Lake Level Target

Standard Lake Level Target

Modelled and Target Lake Water Levels

0.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.80

0.85

0.90

0.95

1.00

18-A

pr-

12

16-M

ay-1

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13-J

un-1

2

11-J

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08-A

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2

05-S

ep-1

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03-O

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ct-

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ov-1

2

26-D

ec-1

2

23-J

an-1

3

20-F

eb-1

3

20-M

ar-

13

17-A

pr-

13

Wate

r L

evel

(mA

HD

)

Lake Alexandrina (Quarterly Lake Cycle)

Lake Albert (Quarterly Lake Cycle)

Quartely Lake Cycling Target Level


Conclusion


4 Conclusion

BMT WBM was commissioned by the South Australian Department of Environment, Water and

Natural Resources (DEWNR) to undertake a range of studies aimed at improving the

understanding of salinity transport and mixing mechanisms in Lake Albert. A numerical model

capable of simulating salinity dynamics of the Lower Lakes and Coorong was used to evaluate the

effectiveness of six potential management options designed to reduce salinity levels in Lake Albert.

The six management options evaluated in the study included:

a) Base Case (i.e. do nothing);

b) Remove Causeway;

c) Dredge Narrung Narrows;

d) Permanent Water Level Control Structure at Narrung;

e) Summer Lake Water Level Cycling; and

f) Coorong Connector.

An initial investigation into the effectiveness of the six management options in reducing salinity in

Lake Albert was undertaken using a 12 month model simulation. The calibrated model was

successfully applied to demonstrate the potential benefits (or otherwise) of the management

options considered.

An examination of the results indicates the following:

Under the Base Case (i.e. “do nothing”) scenario condition, salinity in Lake Albert would

decrease from approximately 5000 µS/cm to approximately 4500 µS/cm in a year.

The Coorong Connector option is clearly the most efficient option for reducing salt concentration

in Lake Albert. At the end of the 12 month simulation, the salt concentration in Lake Albert has

been reduced from ~5000 µS/cm to below 2000 µS/cm.

The Narrung Regulator option is the least efficient option for reducing salt concentration in Lake

Albert with any gate closure reducing the likelihood for wind driven salt export to occur.

The Summer Lake Cycle scenario is capable of removing nearly 40% more salt than the Base

Case scenario and reduces salinity from ~5000 µS/cm to below 4275 µS/cm in a year. If a

Quarterly Lake Cycle water level target was adopted 80% more salt than the Base Case would

be exported over the 12 months with salinity in the centre of Lake Albert falling from

~5000 µS/cm to below 4000 µS/cm in a year.

The “Dredge Narrung” option performs slightly worse than the “Base Case” scenario, while the

“Remove Causeway” option performs slightly better.

This initial testing has only been undertaken for a single set of environmental conditions (i.e. inflow,

evaporation, wind and tides). To gain a better understanding on the envelope of likely future salinity

levels in Lake Albert further modelling was undertaken and presented in BMT WBM (2013c) and

summarised in Section 1.1.5.


References


5 References

BMT WBM (2011a), Data Analysis for the Lower Murray Region – Tides, Winds and Waves,

R.N1874.001.00_DataAnalysis.pdf, Produced for: DENR, June 2011.

BMT WBM (2011b), CLLMM Forecast Model Development – Model Calibration Report,

R.N1874.003.00_ModelCalibration_FinalDraft.pdf, Produced for: DENR, September 2011.

BMT WBM (2011c), CLLMM Forecast Model Development – Development and Benchmarking of

Automated Barrage Logic, R.N1874.006.00_AutoBarrages_Draft.pdf, Produced for: DENR,

October 2011.

BMT WBM (2011i), Lower Lakes, Coorong and Murray Mouth - Modelling of Environmental Water

Requirement and Fully Open Barrage Scenarios, R.N1874.002.01_16SimulationsFinalReport.pdf,

Produced for: DENR, August 2011.

BMT WBM (2012a), CLLMM Forecast Model Development – Model Validation (May – November

2011) Report, R.N1874.008.01_OngoingCalibration_FinalDraft.pdf, Produced for: DENR, February

2012.

BMT WBM (2012b), CLLMM Forecast Modelling – 12 Month Forecast Report,

R.N2228.003.01_12monthForecasts_Draft.docx, Produced for: DENR, June 2012.

BMT WBM (2013a), Lake Albert Salinity Reduction Study - Report on Preliminary Investigations,

Report, R.N20056.001.02_LakeAlbertPrelimInvestigations_Final.docx, Produced for: DEWNR,

June 2013.

BMT WBM (2013b), Lake Albert Salinity Reduction Study – Model Setup and Calibration Report,

R.N20056.002.00_ModelCalibration_Draft_v1a.docx, Produced for: DEWNR, January 2014.

BMT WBM (2013c), Lake Albert Salinity Reduction Study - Three Year Scenario Model

Investigations, R.N20056.004.01_LakeAlbert3yrScenarioModelling_Draft_wAppendixB_v2.1.docx,

Produced for: DEWNR, November 2013.

BMT WBM Bangalow 6/20 Byron Street Bangalow 2479 Tel +61 2 6687 0466 Fax +61 2 66870422 Email [email protected] Web www.bmtwml.com.au

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